There is a growing trend toward smaller and smaller components for electrical applications. Thus micro-electromechanical (MEM) systems and microsystems technology (MST) have made rapid progress in recent years. MEMS/MST technology has the advantages of reliability, small size, low weight and low cost. MEMS is probably best known for its sensor and actuator applications.
The substrate material most used in the production of micro-electronic circuitry is silicon (Si). Among other suitable materials used are silicon dioxide (SiO2), silicon nitride (SiN), polycrystalline silicon, and quartz.
FIG. 1a is an explosion view of a prior-art acceleration sensor based on the inertia of a silicon proof mass. The acceleration sensor has a multilayer structure comprising silicon layers 101, 111, 121 and glass insulator layers 102 and 122, of which the former is located between silicon layers 101 and 111 and the latter between silicon layers 111 and 121. The structure further comprises two stationary capacitor plates, of which the first plate (not shown in the figure) is between a glass insulator layer 102 and a silicon layer 111 and the second plate 124 is between the silicon layer 111 and a glass insulator layer 122. Silicon proof mass 104 is fastened to the frame 111 via two elastic silicon springs 105. Between the proof mass and each glass insulator there is typically a one-micrometer space.
When the acceleration sensor is subjected to acceleration, elastic springs 105 balance the inertia of the proof mass by bending. The relatively small displacement of the inertial mass is measured by comparing the capacitance formed by the second plate 124 and the proof mass with the capacitance formed by the first plate and the proof mass. The electrical connections needed for said comparison are formed by metal films 103, 113, and 123 arranged on corresponding outer surfaces of each of the silicon layers.
An accelerometer such as the one depicted in FIG. 1a can easily be constructed to measure the desired acceleration range. However, drawbacks are that it is expensive to manufacture and requires complicated measurement electronics needing a power supply.
FIG. 1b depicts a top view of a prior art latching accelerometer that mechanically records shocks without needing a power supply. Such a latching accelerometer can be used as a peak-reading shock recorder typically covering the range from 60 g to 3500 g (g is acceleration of gravity).
The main parts of the accelerometer are a pedestal 151, a flexible cantilever 153 with one end fixed to the pedestal, an inertial mass 152 in the middle of the cantilever, and a number of notches 154-158 forming an arc. Movement of the cantilever is prevented by dose glass and/or silicon surfaces (159 and 160) on either side of the cantilever in the plane parallel to the paper.
When the accelerometer is subjected to acceleration the inertial mass deflects the cantilever tip 158. Depending on the amount of acceleration the tip moves from one notch to another. For example, if the original position of the tip is between notches 155 and 156, it may move to between notches 154 and 155 or alternatively to between notches 156 and 157.
The spring force of the elastic cantilever (dimensions are typically: length 1 mm and thickness 5 μm) is not strong enough to return the tip to the original position. Additional stops 161 and 162 can be arranged that allow incremental thresholds to be recorded.
Although this accelerometer is adequate for many purposes, typically as indicators of rough handling in shipping operations according to an editorial article in Electronic Design Magazine of Jun. 23, 1997, pp. 28-31, the mechanically latching accelerometer has several drawbacks. Due to friction the acceleration threshold is hard to control precisely, and variation between individual units is high. Additionally, the inertial mass indicates acceleration in one plane only.
Most of the MEMS accelerometers developed are based on an inertial proof mass, which acts on a spring or springs, and the deflection from the idle position is measured. For example, a capacitive circuit element can be made to change capacitance depending on this deflection. Automobile air-bag accelerometers typically use this measurement method and have been developed into reliable mass-produced low-cost devices.
Micromechanical lateral field emitters arranged on bending cantilevers are used in some acceleration sensors. In such sensors the strength of current is based on the bending of the cantilever, which deflects lateral field emitters from opposing each other. The acceleration sensor needs supporting electronics in both said prior-art cases. However, this increases the production costs, which is not acceptable in many cases.
Prior-art acceleration sensors are generally discrete devices where the sensor and the measurement electronics are implemented on separate chips. Lately some surface micromachined acceleration sensors have been devised, where the sensor and the measurement electronics are implemented on a single semiconductor chip. Such sensors are generally packaged in single chip modules (SCM) or multichip modules (MCM) featuring both the sensor and the measurement electronics in the same package.
One drawback is that at the moment there is no such sensor commercially available, suitable for mass-produced handheld terminals, such as electronic books, with the capability to register or warn when the terminal has suffered an acceleration shock. Additionally, no method is provided for remote reading in the prior-art sensors, or for time registering shock events.
Normal practice is that companies provide a warranty for products such as electronic equipments. If any faults or defects are found during a warranty period, the customer has the right to claim either repair or replacement of the faulty equipment free of charge. However, there is no method to find out whether the customer has handled the electronic device too roughly or whether the device was already damaged when received. Usually the product itself does not in any way inform either the user or the repairman of mishandling if no visible physical damage can be found. Unnecessary warranty repairs in consequence of mishandling are common today due to the fact that the cause of breakage or damage is untraceable. From the manufacturers' and dealers' point of view this is frustrating and often very uneconomical. Mishandling could be minimized if the product in one way or another warned the user of rough usage which could damage the product.
Furthermore, illegitimate warranty claims could be avoided if the product itself could indicate abusive handling. Mishandling of lent equipment could also be avoided if the borrower knows that any mishandling can be ascertained when the equipment is returned.